Transplantation of marrow form normal animals to lethally irradiated animals provides an ideal situation for evaluating the basic physiologic aspects of hemopoeitic renewal fand further defining primitive stem cells and their regulators. Theoretically, increasing the rapidity of hemopoietic renewal after transplantation would improve survival and lessen morbidity. We propose to examine approaches to enhance marrow recovery using hypertransfusion of marrow cells ("marrow doping") and "alternative" sources of marrow ("super marrow") transplanted to normal irradiated host mice. A number of in vivo manipulations in different animal systems leads to enhanced marrow recovery without marrow transplantation and may provide "alternative" sources of marrow to use for transplantation. These include marrow from animals 1) treated with low dose radiation or chemotherapy, 2) stimulated with endotoxin or lithium carbonate 3) pretreated with 5 FU which generates high proliferative potential colony forming cells (HPP-CDC) as described by Bradley. Although it has been widely held that transfused bone marrow cells will not proliferate in normal, untreated mice, successful engraftment has been achieved when large numbers of nucleated marrow cells are transfused to normal mice. By increasing the baseline number of stem cells prior to radiation or chemotherapy it may be possible to protect animals by shortening the time period for full hemopoietic reconstitution. We also propose to evaluate the creation of "super marrow" in vitro by the addition of various drugs or agents affecting primitive stem cell growth including the addition of 5 FU, lithium chloride or endotoxin to normal murine marrow. Also, interleukin-3 has been shown to support the growth of primitive stem cells, specifically Thy-1 + post 5 FU cells in liquid culture. In addition a synergistic activity produced by a marrow adherent cell line, TC-1, supports liquid culture growth of hemopoietic stem cells along with pre B cell differentiation. Thus, exposure of normal marrow or 5 FU marrow to TC-1 conditioned media, IL-3, lithium chloride, or endotoxin in liquid culture including Dexter or Whitlock-Witte cultures may provide in vitro sources of stem cells with enhanced marrow renewal capacity. For in vivo studies using irradiated host mice, survival curves, stem cell renewal capacity and soft agar clonagenic stem cell assays will be determined.